The prior art teaches disc brakes for motor vehicles generally comprising a circular disc or rotor mounted to a wheel for rotation therewith about an axis. An anchor member, rigidly mounted to the axle which supports the wheel, also provides rigid support for a caliper which otherwise straddles a portion of the periphery of the rotor. Brake linings or "pads" supported by the caliper on either side of the disc are urged into engagement with frictional surfaces on the opposite faces of the rotor to generate braking torque, as by supplying pressurized hydraulic fluid to a piston housed within one or both ends of the caliper.
Typically, these prior art disc brakes utilize like-sized brake pads on either side of the rotor. Each brake pad is likewise provided with substantially the same effective radius, that is, the braking force generated by each brake pad upon being urged against its respective mating rotor surface is effectively offset the same radial distance from the rotational axis of the wheel/rotor assembly. The contact area of each brake pad is typically "aligned" with, i.e., is centered about, the longitudinal axis of the piston operating to urge that pad towards its mating rotor surface. The normal forces exerted by the brake pads on both rotor surfaces will thus likewise be axially aligned and, hence, application of an undesirable moment on the brake rotor will be avoided (the art has recognized that application of such moments are undesirable, for example, given the likelihood of uneven wear of the brake pads).
However, in at least one prior art design for a "fixed"-type caliper--that is, a nonswinging dual-opposed-piston caliper--the pad located on the side of the rotor nearest the hub of the wheel (hereinafter "the outboard pad") is deliberately positioned nearer the wheel's rotational axis than its counterpart on the other side of the rotor (hereinafter the "inboard pad"). This is an attempt to provide a maximum effective radius/swept area for each individual pad while otherwise accommodating those wheels whose inner rims provide less radial clearance for the outboard pad than for the inboard pad. In such a design, the outboard portions of the caliper, including its outboard piston's longitudinal axis, are likewise displaced radially inwardly, relative to the inboard portions of the caliper, towards the wheel's rotational axis. As might be expected, under this approach, each like-sized brake pad is maintained in alignment with the longitudinal axis of its respective piston.
Unfortunately, while affording greater radial clearance between the wheel rim and the periphery of the brake caliper--particularly along its outboard end--the radially inward displacement of the outboard pad and its driving piston correlatively reduces the pad's effective radius. Since the effective radius of the inboard pad remains unchanged, the resulting disparity between the effective radius of each pad, coupled with the lack of alignment between the longitudinal axes of the inboard and outboard pistons, results in application of a bending moment to the rotor and, correspondingly, uneven brake pad wear. This, in turn, results in a loss over time of vehicle braking performance, offsetting any potential gains which might otherwise have resulted from the increased swept area achieved by the radially-outer inboard pad.
What is needed, then, is an improved caliper-type disc brake for the wheel of a motor vehicle which maximizes use of available packaging space within the wheel's inner rim to boost vehicle braking performance while minimizing applied moments and, correlatively, the uneven wear likely to result therefrom.